1. Field of the Invention
This invention relates to an inflation pressure indicator for vehicle tires. The term vehicles includes, inter alia, passenger vehicles, trucks, tractors, motor cycles, aircraft and other vehicles fitted with pneumatic type tires.
2. Description of the Prior Art
For maximum safety, the tires of automobiles and other vehicles must be maintained within defined inflation pressure limits set down by the tire manufacturers. If the tires are operated when under-inflated, the excess heat generated in the tire casing as it flexes may cause the casing to fail, while over-inflation of the tire reduces the effective contact area between the tire tread and the pavement and the risk of blowouts is also markedly increased.
Conventionally, it has been necessary to use a separate tire pressure gauge to check the inflation pressure in the vehicle tires. However, tests by motoring organizations have shown that the pressure gauges provided on the forecourts of garages and service stations are often inaccurate and errors of 14-28 Kpa (2-4 p.s.i) have been recorded.
One proposal to overcome this problem was to provide each vehicle tire with a cap which screwed onto the tire stem. The cap opened the tire valve and the pressure in the tire operated a piston which had a piston rod graduated in Kpa (psi) to indicate the inflation pressure. This type of cap had a number of disadvantages. Firstly, the readings given were often inaccurate and the piston rod often became stuck due to the ingress of dust into the cap. Secondly, air leaks developed in the cap and the tires became deflated. Thirdly, the cap had to be removed from the valve to enable the tires to be inflated. Because of these and other problems, these caps were found to be not acceptable.